1. Field of the Invention
The present invention relates to a sheet conveying apparatus, an image forming apparatus and an image reading apparatus.
2. Related Background Art
Some of sheet conveying apparatuses for conveying sheets are respectively provided with skew conveying correction means which is means for correcting skew conveying correction and position deviation of a sheet.
A sheet conveying apparatus is provided for image formation and image reading of a copying machine, printer, facsimile or scanner. Skew conveying correction means is used immediately before an image forming portion or image reading portion to adjust the attitude and position of a sheet.
In this case, as a system of the skew conveying correction means, there is a loop forming system for forming a loop on a sheet by running a front end of a sheet against a nip of a stopped roller pair. In the case of the sheet whose front end is run against the roller pair, the skew conveying is corrected because the front end of the sheet goes along the nip of the roller pair.
However, the above loop forming system requires a loop space for forming a loop and an apparatus is increased in size.
When the loop space cannot be sufficiently secured, there is a problem that jam due to buckling occurs in weak-kneed thin paper.
There is a problem that sound occurs when bringing a sheet into contact with a roller pair.
There is a problem that a skew conveying correction capacity is changed due to the strength of the knee of a sheet. In the case of thin paper having no knee of sheet, the contact pressure when the front end of the sheet contacts with the nip of a roller pair becomes insufficient and thereby the front end of the sheet may not sufficiently contact with a resist roller pair. In this case, skew conveying correction cannot be completely made. However, in the case of thick paper having strong knee of sheet, a trouble may occur that the paper penetrates the nip of the roller pair due to an impact when the paper contacts with the nip portion of the roller pair.
Moreover, when there is a curl or broken portion on the front end of a sheet, the front end of the sheet does not accurately follow the nip portion of a roller pair and a case occurs in which skew conveying correction cannot be accurately made. Therefore, this is not sufficiently satisfied as a printing accuracy.
Furthermore, a substantial image forming rate is improved without raising a process speed for image formation in the case of image formation by decreasing the interval between sheets (sheet interval) and processing many sheets in a short time because an image forming apparatus and image reading apparatus are recently digitized.
In the case of the previously-described loop forming system, it is necessary to once stop a sheet to form a loop. Therefore, the interval between sheets (sheet interval) is inevitably decided and it is greatly impeded that the productivity of substantial sheet conveying is improved.
A sheet conveying apparatus is proposed which makes it possible to automatically cure a skew of a sheet due to skew conveying of the sheet (refer to Japanese Patent Application Laid-Open Nos. H10-067448 and 2001-273538).
The apparatus disclosed in Japanese Patent Application Laid-Open No. H10-067448 is provided with a conveying roller pair (skew conveying correction rollers) for nipping and conveying a sheet and a skew-amount detecting sensor for detecting a skew amount of a sheet set at the downstream side in the conveying direction of the conveying rollers. The skew conveying of the sheet is corrected by displacing the conveying rollers in accordance with the skew of the sheet in accordance with the information of the sheet-skew-amount detecting sensor.
In the case of this sheet conveying apparatus, however, conveying rollers are skewed from the original sheet conveying direction when the skew conveying correction of the sheet is performed as described above. Therefore, when conveying a sheet under this state, the sheet is conveyed in a direction skewed from the original sheet conveying direction (hereafter referred to as skew feeding). In this case, when applying the sheet conveying apparatus to an image forming apparatus, an image is transferred by being shifted from the sheet by an amount for performing skew feeding in the direction orthogonal to the sheet conveying direction and there is a trouble that a printing accuracy is extremely deteriorated.
To correspond to the trouble of skew feeding, in the case of Japanese Patent Application Laid-Open No. H10-067448, a sensor for detecting the position of an end of a sheet in the direction orthogonal to the sheet conveying direction at the side end of the sheet is set to the downstream side of conveying rollers. Moreover, a conveying roller pair is moved in the direction orthogonal to the sheet conveying direction to correct a position in accordance with a detection result by a sheet-end-position detecting sensor.
Furthermore, in the case of the bill processor disclosed in Japanese Patent Application Laid-Open No. 2001-273538, two sets of correction roller pairs to be pivotally moved about a pivotal movement axis extended in the face direction of a bill conveyed along a conveying route are set. By pivotally moving and skewing the upstream-side correction roller pair and skew-feeding a bill by the upstream-side correction roller pair, a displacement in the width direction intersecting with the conveying direction of the bill is corrected. Thereafter, the bill is held by the-downstream-side correction roller pair and then, the skew (angular shift) of the bill is corrected by skewing the downstream-side roller pair.
When using a skew correcting mechanism for displacing a conveying roller pair so as to skew it from the direction orthogonal to a sheet conveying direction and a position correcting mechanism for moving the conveying roller pair in the direction orthogonal to the sheet conveying direction like the configuration disclosed in Japanese Patent Application Laid-Open No. H10-067448, an apparatus is increased in size and the cost is increased. Sufficient time is necessary in order to perform a series of correcting operations because positional correction in the direction orthogonal to the sheet conveying direction is performed. Therefore, it is difficult to increase the conveying speed of a sheet in conveying rollers in order to increase the productivity of sheet conveying.
Moreover, the configuration disclosed in Japanese Patent Application Laid-Open No. 2001-273538 skews a correction roller pair at downstream side after nipping a bill by the downstream-side correction roller pair. The bill is skew-fed by the downstream-side correction rollers and a displacement in the width direction occurs. Moreover, it is necessary to use two sets of correction roller pairs which pivotally move about the pivotal movement axis extend in the face direction of the bill and an apparatus is increased in size and the cost is increased.